T his automatic bicycle light makes cycling in the dark much easier (although you still need to pedal of course). The circuit takes the ambient light level into account and only turns on the light when it becomes dark. The light is turned off when no cycling has taken place for over a minute or if it becomes light again. The biggest advantage of this circuit is that it has no manual controls. This way you can never ‘forget’ to turn the light on or off. This makes it ideal for children and those of a forgetful disposition
To detect when the bicycle is used (in other words, when the wheels turn), the circuit uses a reed switch (S1), mounted on the frame close to the wheel. A small magnet is fixed to the spokes (similar to that used with most bicycle speedometers), which closes the reed switch once for every revolution of the wheel. Whilst the wheel turns, pulses are fed to the base of T1 via C1. This charges a small electrolytic capacitor (C2). When it is dark enough and the LDR there-fore has a high resistance, T2 starts conducting and the lamp is turned on. With every revolution of the wheel C2 is charged up again. The charge in C2 ensures that T2 keeps conducting for about a minute after the wheel stops turning. Almost any type of light can be connected to the output of the circuit.